There are always quite a few games that dont work well in sli/crossfire.
A single card is always more stable than dual cards in sli/crossfire, and i mean always.
FPS/dollar is a silly approach. They dont take into account the fps variation (double the cards double the variation) which will tarnish your game experience, the power requirements, the upgradeability, the stability, and last but not least dont forget you pay for 2 cards but only get 1.5 times the power at best.
SLI/Crossfire is great for stores and manufacturers (they sell 2 crap cards instead of one) but that doesnt mean its good for you.
I know the stores and the manufacturers ( and their reviewer lackeys) would like to make you think that two low cards are better than one high card, but thats not the case, unfortunately.
You save a very small amount by going SLI but lose a lot in the process.
If you dont play games but only run benchmarks on your pc, then go dual-card, if you play games and like to play instead of spending time on tech support forums, go single card.

Edit: and avoid Nvidia 5xx cards, go for the 4xx. My gtx 480 works 10 times better than the gtx 580 (both from asus btw). I never intended to own both cards (long story) but now that i do, i got insight into how bad the 5xx architecture really is. Shame on you Nvidia, i always been a fan, not anymore.
Also it was a good insight into how dishonest and/or uninformed the reviews are.